Ferroelectric thin film dielectrics have applications in integrated circuits, for example, as ferroelectric capacitor elements as used in ferroelectric DRAMs. Ferroelectric dielectric materials for integrated circuit applications are typically complex metal oxides of the perovskite structure, of which barium strontium titanate and lead zirconate titanate are well known examples. The materials may be used undoped, or doped, e.g. lead lanthanum zirconate titanate (PLZT). The functional properties of these ferroelectric materials are typically dependent on the nature of the substrate on which they are formed, that is, the material forming conductive electrodes of the capacitor element. In particular, the electrode material selected must resist oxidation under the processing conditions used for formation of the ferroelectric material. Processes for formation of ferroelectric dielectrics may involve processing/annealing in oxidizing ambients, e.g. oxygen and ozone. Thus, conventional electrode materials including polysilicon, aluminum, titanium are unsuitable because they tend to oxidize and/or react with ferroelectrics, such as PZT, under these conditions. Noble metals, particularly platinum, have thus been the most successful bottom electrode material, although the use of gold is often restricted in silicon IC processing.
The adhesion of ferroelectric films such as PZT to platinum is reasonably good when the Pt forms a bottom electrode of a capacitor. That is, when a ferroelectric precursor is deposited on a Pt electrode, the Pt layer provides the initial support as the nucleation centre for crystallization of a ferroelectric phase of the material, and adhesion is satisfactory. However, after crystallization of the ferroelectric layer, adhesion of a top electrode of platinum formed on the processed PZT is poor. The sequence of processing means that the interface between the bottom electrode and the PZT and between the PZT and the top electrode are quite different, and thus the capacitor structure is not symmetric.
Often, the Pt layer forming the top electrode tends to lift during processing, and electrical characteristics are poor or unreproducible. While some other metals, e.g. Al and Ti, adhere better to PZT, they tend to oxidize at the interface, degrading electrical characteristics.